First, after you've added some friends to the game, you're going to hover your mouse over their profile picture and choose "VISIT":

After you enter their world, you can scroll around and see what kind of weapons they have and what other regions they have unlocked. The cool part about this is that you can battle in any of their pending battles by clicking on an area that has a bouncing arrow above it, and not have it cost any energy:

These battles are a bit different from a normal battle though…you'll just have a single weapon to use to try and take out the enemy at that location. If you can muster the courage and strategy to defeat the battle, you get a choice at the end of the battle:

If you are greedy, you can just take extra loot and resources for yourself, not helping your friend in the slightest. If you want to be a helpful friend though, you can leave a "marker" behind for your friend that will automatically show them the placement of one of the enemy ships when they fight that battle!

Make sure to let your friend know your decision too!